Conventionally, and to improve the yield of a plantation, only parameters external to the actual plants, basically such as climate and irrigation of the soil, were examined and considered.
However, and since some time, it is known that plants undergo contractions/dilations throughout the day, namely an increase and decrease of their diameter (in the case of the trunk or of a fruit) that is a function of its “hydric” condition, of its “stress”, so that a dendrometric and permanent control of the plant allows decisions much more adapted to improving the yield to be taken, than if only parameters external to it are considered.
Evidently these dimensional changes are minimal because of which the dendrometer used must be of great precision.
One well-known and commonly used solution in this respect consists of so-called linear movement sensors, structured by means of an electromagnetic ring-shaped coil, in whose cavity a nucleus formed by a rod freely moves, that, due to the effect of the magnetic field of the coil, is kept constantly pressed against the surface of the plant, such as, for example, against the trunk of a tree, so that when this latter expands or contracts (a few microns a day), the nucleus or rod undergoes the equivalent axial movement generating a modification in the field of the coil, that can be perfectly measured and, using suitable conversion tables, transformed into a measurement of length.
Also known are sensors or dendrometers based on the use of micrometrical gauges or extension measurement bands, based in turn in the fact that in all materials that are conductors of electricity a proportion exists between the length of said conductive and its electrical resistance, so that a proportion can be obtained between the relative change of length of a conductor and the change in its electrical resistance, that allows the dimensional increase or decrease of the plant being analysed to be deduced.
In this area the U.S. Pat. No. 4,549,355 patent should be noted, in which a Wheatstone Bridge is used as a measuring element, in which four electrical resistances connected between four points or ends of the bridge are used, which is supplied at two points to produce an output voltage in the other two that must be zero when the bridge is in balance, in such a way that this outlet voltage of value zero will vary becoming positive or negative depending on whether an increase in one or other of the resistances, that form part of the Wheatstone Bridge, occurs.
This solution, perfectly valid from the theoretical viewpoint, in practice presents problems arising from the geometry of the plant, so that the orientation in the deformation of the plant is usually neither aligned with the electro-resistant element, nor does it offer a fixed angulation, because of which differences exist between the measurement taken and the real value that can become unacceptable.